package com.baidu.interview;

import java.util.Scanner;

/**
 * @author ZhangyiA
 * @describe: n 个学生 , m 个食堂 , 每个食堂 k 个窗口
 *            学生选择每个食堂的概率相同 ， 在排队过程中，学生倾向于是最长队伍尽可能短
 *            现求 计算出所有食堂内队伍长度的期望
 * @date 2021/07/03
 */

public class Three_LineUpForDinner {

    private double calc(int[][] num ,int x , int y){
        if(x < y) return 0;
        return num[x][y];
    }

    public static void main(String[] args){
        Scanner input = new Scanner(System.in);
        Three_LineUpForDinner dinner = new Three_LineUpForDinner();
        int[][][] dp = new int[51][51][51];
        int[][] num = new int[51][51];
        int n = input.nextInt();
        int m = input.nextInt();
        for(int i = 0 ; i <= n ; i ++){
            num[i][0] = 1;
        }
        for(int i = 1 ; i <= n ; i ++){
            for(int j = 1 ; j <= i ; j ++){
                num[i][j] = num[i - 1][j - 1] + num[i - 1][j];
            }
        }
        int[] a = new int[m + 1];
        for(int i = 1 ; i <= m ; i ++){
            a[i] = input.nextInt();
        }
        for(int i = 0 ; i <= m ; i ++){
            dp[i][0][0] = 1;
        }

        for(int i = 1 ; i <= m ; i ++){
            for(int j = 1 ; j <= n ; j ++){
                for(int k = 1 ; k <= n ; k ++){
                    for(int p = (k - 1) * a[i] + 1 ; p <= k * a[i] ; p ++){
                        if(p <= j){
                            for(int l = 0 ; l <= k ; l ++){
                                dp[i][j][k] += dp[i - 1][j - p][l] * dinner.calc(num , n - j + p , p);
                            }
                        }
                    }
                    for(int p = 0 ; p <= (k - 1) * a[i] ; p ++){
                        if(j >= p) dp[i][j][k] += dp[i - 1][j - p][k] * dinner.calc(num , n - j + p , p);
                    }
                }
            }
        }
        double ans = 0;
        for(int i = 1 ; i <= n ; i ++){
            ans += dp[m][n][i] * 1.0 * i;
        }
        for(int i = 1 ; i <= n ; i ++){
            ans /= m;
        }
        System.out.println(String.format("%.10f" , ans));
    }

}
